Peel and finely dice the onions and garlic. Peel carrot and celery and cut into fine cubes. Wash oregano, shake dry and chop finely. Heat oil in a pot and fry half of the onions, garlic, carrot and celery for about 2 minutes.
Add tomatoes and season with oregano, salt and pepper. Put the sauce in an ovenproof dish and bake in a preheated oven (electric cooker: 150 °C/ convection oven: 125 °C/ gas: see manufacturer) for 35 minutes.
Wash parsley, shake dry and chop finely. Mix the remaining onion cubes, minced meat, breadcrumbs, tomato paste, egg and parsley. Season with salt and pepper. Form small balls out of the minced mass.
Heat the oil in a pan. Fry the meatballs in it while turning them. Grate cheese. Add the meatballs to the sauce about 12 minutes before the end of the cooking time and sprinkle with the cheese.